We sought to delineate the pharmacokinetics (PK) of oseltamivir and its active metabolite oseltamivir carboxylate during pregnancy. Physiologic changes of pregnancy, including increased renal filtration and secretion, may increase the clearance of oseltamivir carboxylate. Sixteen pregnant women taking oseltamivir for prophylaxis or treatment of suspected/proven influenza infection were enrolled. Twenty-three nonpregnant reproductive-age females served as the control group. The primary PK endpoint was area under the plasma concentration time curve for oseltamivir carboxylate. Pregnancy did not alter the PK parameters of the parent compound, oseltamivir. However, for oseltamivir carboxylate the area under the plasma concentration time curve was significantly lower (P = .007) and the apparent clearance significantly higher (P = .006) in pregnant women compared with nonpregnant women. Pregnancy produces lower systemic levels of oseltamivir carboxylate. Increasing the dose and/or dosing frequency of oseltamivir during pregnancy may be necessary to achieve comparable exposure in pregnant and nonpregnant women.